1. Field of the Disclosure
The present disclosure is related to absorber tubes. More particularly, the present disclosure is related to absorber tubes having a central metal tube and a glass cladding tube, which surrounds the central metal tube.
2. Description of Related Art
DE 10 231 467 B4 describes such an absorber tube, which, in particular, is employed for parabolic trough collectors in solar technology power plants. The absorber tube comprises a central metal tube and a glass cladding tube surrounding the central metal tube. The glass tube is provided with a glass-metal transition element at both ends, at each of which an expansion compensation device or an attachment element engages. The expansion compensation device is at least partially arranged in the annular space between the metal tube and the glass transition element.
The attachment element can extend between the expansion compensation device and the metal tube in the annular space, as a result of which, low-angle incident radiation as well as emitted radiation of the metal tube are reflected back onto the metal tube in the case of a conical configuration of the attachment element.
The attachment element can also extend between the expansion compensation device and the glass tube in the annular space and, in this case, it is joined to the glass-metal transition element. The attachment element has a fastening element in the form of an annular disc, with which the attachment element is fastened to the expansion compensation device, which can include a bellows.
Known from DE 60 223 711 T2 is an absorber tube in which the glass-metal transition element and the expansion compensation device in the form of a bellows are arranged one behind the other in the axial direction. Provided on the outer side are a first shielding element, which shields the bellows, and a second shielding element, which shields the glass-metal transition element against incident radiation. Moreover, an internal radiation shield is arranged between the cladding tube and the metal tube in the region of the glass-metal transition element in the annular space. The internal radiation shield is suspended by retaining arms in the bellows.
A drawback of this arrangement with a bellows having a glass-metal transition element is the relative long length of the structure, which leads to a reduction in the free aperture and thus of the efficiency.
The glass-metal transition element is initially shielded from the outside against direct irradiation by one of the two outer shielding elements. The inner radiation shield, which has an L-shaped cross section, has only support arms in the region of the glass-metal transition element, so that low-angle incident radiation and radiation reflected from the metal tube can impinge in the region between the support arms on the glass-metal transition element. Only a part of this radiation is intercepted by the inner radiation shield.
This design has the further drawback that it itself absorbs radiation and, as a result, is heated. The inner radiation shield is thermally coupled only insufficiently to the bellows via the support arms, so that the portion of the radiation that is absorbed by the annular component has to be emitted again in large part by radiation. A substantial portion of the radiated heat impinges in turn on the glass-metal transition element. As a result of this, the glass-metal transition element undergoes a secondary input of heat due to radiation emitted by the heated radiation shield.
Both DE 10231467 B4 and DE 60223711 T2 disclose an outer-lying protection of the glass-metal transition and the expansion compensation device. This outer-lying protection is nowadays generally mounted in the power plant after installation of the receiver. To this end, sheet-metal reflectors are mounted after welding together the receiver in the field.
A drawback of this is that the glass-metal transition is unprotected during transport and installation and, as a result, can readily be damaged. During the mounting process, scratches can occur in the cladding tube, with scratches in the vicinity of the glass-metal transition element being especially critical and possibly leading to weakening of the absorber tube and later glass fracture during operation.